Java 8 中的List排序

柔情只为你懂 2022-01-15 19:49 417阅读 0赞

*  按字母顺序排序字符串列表

List<String> cities = Arrays.asList(
           "Milan",
           "london",
           "San Francisco",
           "Tokyo",
           "New Delhi"
    );
    System.out.println(cities);
    //[Milan, london, San Francisco, Tokyo, New Delhi]
    
    cities.sort(String.CASE_INSENSITIVE_ORDER);
    System.out.println(cities);
    //[london, Milan, New Delhi, San Francisco, Tokyo]
    
    cities.sort(Comparator.naturalOrder());
    System.out.println(cities);
    //[Milan, New Delhi, San Francisco, Tokyo, london]

*Java 7*中，我们使用`Collections.sort`对List进行排序，`Collections.sort`方法参数为`List`，`Java 8`中新增了一个集合方法`List.sort`，方法接受参数`Comparator`

*  排序整数集合

List<Integer> numbers = Arrays.asList(6, 2, 1, 4, 9);
    System.out.println(numbers); //[6, 2, 1, 4, 9]
    
    numbers.sort(Comparator.naturalOrder());
    System.out.println(numbers); //[1, 2, 4, 6, 9]

*  按某个字符串属性列排序

List<Movie> movies = Arrays.asList(
            new Movie("Lord of the rings"),
            new Movie("Back to the future"),
            new Movie("Carlito's way"),
            new Movie("Pulp fiction"));
    
    movies.sort(Comparator.comparing(Movie::getTitle));
    
    movies.forEach(System.out::println);

结果输出

Movie{title='Back to the future'}
    Movie{title='Carlito's way'}
    Movie{title='Lord of the rings'}
    Movie{title='Pulp fiction'}

我们注意到，给`sort`方法传递的并非`Comparator`类型，但结果仍然正确排序，原因就在于我们比较的属性列是`String`，而`String`已经实现了`Comparable`接口，查看`Comparator.comparing()`的定义实现，我们可以看到其其调用的就是比较关键字的`compareTo`方法

return (Comparator<T> & Serializable)
                (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));

*  按Double类型排序

List<Movie> movies = Arrays.asList(
            new Movie("Lord of the rings", 8.8),
            new Movie("Back to the future", 8.5),
            new Movie("Carlito's way", 7.9),
            new Movie("Pulp fiction", 8.9));
    
    movies.sort(Comparator.comparingDouble(Movie::getRating)
                          .reversed());
    
    movies.forEach(System.out::println);

本质上，`Comparator.comparingDouble()`调用的还是`Double.compare()`的方法，同样类似的还有`comparingInt()`和`comparingLong()`方法

*  自定义比较规则

比如，对于我们之前提到的`Movie`,这里我们需要按星标排序，那么我们可以自定义`Comparator`实现类

List<Movie> movies = Arrays.asList(
            new Movie("Lord of the rings", 8.8, true),
            new Movie("Back to the future", 8.5, false),
            new Movie("Carlito's way", 7.9, true),
            new Movie("Pulp fiction", 8.9, false));
    
    movies.sort(new Comparator<Movie>() {
        @Override
        public int compare(Movie m1, Movie m2) {
            if(m1.getStarred() == m2.getStarred()){
                return 0;
            }
            return m1.getStarred() ? -1 : 1;
         }
    });
    
    movies.forEach(System.out::println);

得到输出结果如下：

当然，我们也可以**Lambda**表达式以匿名类简写如下：

movies.sort((m1, m2) -> {
        if(m1.getStarred() == m2.getStarred()){
            return 0;
        }
        return m1.getStarred() ? -1 : 1;
    });

或者通过`Comparator.comparing()`方法

movies.sort(Comparator.comparing(Movie::getStarred, (star1, star2) -> {
        if(star1 == star2){
             return 0;
        }
        return star1 ? -1 : 1;
    }));

*  按`chain of Comparator`排序集合

还是上面的例子，如果我们需要首先按电影星标，然后再按评分高低排序，那么我们可以通过如下方式实现。

List<Movie> movies = Arrays.asList(
            new Movie("Lord of the rings", 8.8, true),
            new Movie("Back to the future", 8.5, false),
            new Movie("Carlito's way", 7.9, true),
            new Movie("Pulp fiction", 8.9, false));
    
    movies.sort(Comparator.comparing(Movie::getStarred)
                          .reversed()
                          .thenComparing(Comparator.comparing(Movie::getRating)
                          .reversed())
    );
    
    movies.forEach(System.out::println);

我们可以得到排序结果如下：

Movie{starred=true, title='Lord of the rings', rating=8.8}
    Movie{starred=true, title='Carlito's way', rating=7.9}
    Movie{starred=false, title='Pulp fiction', rating=8.9}
    Movie{starred=false, title='Back to the future', rating=8.5}

原文链接：

[https://reversecoding.net/java-8-comparator-how-to-sort-a-list/][https_reversecoding.net_java-8-comparator-how-to-sort-a-list]

[https_reversecoding.net_java-8-comparator-how-to-sort-a-list]: https://reversecoding.net/java-8-comparator-how-to-sort-a-list/